WASHINGTON — The FBI probe of two men arrested in Amsterdam after suspicious items turned up in one of the men's luggage is finding they were probably not on a test run for a future terror attack, a U.S. official said Tuesday, casting doubt on earlier suggestions even as Dutch authorities held the pair on suspicion of conspiring to commit a terrorist act.

The U.S. does not expect to charge the men, a law enforcement official said. The two men arrested in Amsterdam — both traveling to Yemen — did not know each other and were not traveling together, a U.S. government official said.

The officials spoke to The Associated Press on condition of anonymity to discuss the investigation.

The Amsterdam arrests came at a time of heightened alert less than two weeks before the ninth anniversary of the Sept. 11, 2001 terror attacks. U.S. officials have also been concerned about Americans traveling to Yemen to join Al Qaeda.

Before officials began casting doubt on the test run theory, FBI agents were chasing down leads in Detroit, Birmingham, Ala., and Memphis, Tenn., a law enforcement official said.

"These two passengers have not been charged with any crime in the United States and we caution you against jumping to any conclusions," Homeland Security spokeswoman Amy Kudwa said Tuesday.

U.S. officials had earlier said they were investigating whether the two men had been conducting a dry run for a potential terrorist attack. But as the probe evolved, officials said that appeared unlikely.

Both of the detained men missed flights to Washington Dulles International Airport from Chicago, and United Airlines then booked them on the same flight to Amsterdam, the U.S. government official said. The men were sitting near each other on the flight, but not together.

A Dutch prosecutor Tuesday did not address why the two men continued to be held even as U.S. officials cast doubt on the case.

"We are taking it seriously. Otherwise we would not have arrested them," Theo D'Anjou of the Dutch national prosecutor's office said. Anjou added that the Dutch investigation was under way "to see whether we can charge them, and if we can charge them, with what."

Under Dutch law, they can be held for three days and 15 hours from the time of their arrest. After that period, they must be taken before an investigating judge if officials want their custody extended.

A U.S. official identified the men as Ahmed Mohamed Nasser al Soofi and Hezam al Murisi. Al Soofi had a Detroit address. Al Soofi had been living in Tuscaloosa, Ala., and working in a convenience store, the state's homeland security director Jim Walker said. There was nothing that al Soofi had done in Alabama that brought him to the attention of Alabama officials, Walker said.Al Soofi, who was bound to Dubai from Birmingham, Ala., through connections in Chicago and Dulles in northern Virginia, was questioned by the Transportation Security Administration as he went through security in Birmingham on Sunday, one of the officials said.

Al Soofi told the authorities he was carrying a lot of cash. Screeners found $7,000 on him, but he was not breaking any law by carrying that much money. It is not unusual for people to carry large amounts of cash when they travel to Third World countries.

TSA screeners took a closer look at his checked baggage. It was then that they discovered suspicious items in his bag, a cell phone taped to a Pepto-Bismol bottle, multiple cell phones and watches taped together, and a knife and box cutter, according to another U.S. official who had been briefed on the investigation.

None of the checked items violated U.S. security rules, so TSA allowed al Soofi to fly.

Kip Hawley, the former Transportation Security administrator, said this incident is not the first time TSA screeners have seen suspicious items bound together. It is not an everyday occurrence, but screeners are trained to take a second look when they see something suspicious to determine whether it's a dangerous or illicit item that shouldn't be allowed to fly. In 2007, TSA alerted screeners that suspicious items found at U.S. airports may indicate that terrorists were conducting dry runs. Screeners are deliberately on the lookout for such items.

But when al Soofi got to Chicago, he changed his travel plans to take a direct flight to Amsterdam, while his luggage went on to Virginia. When Customs officials discovered he was not on the later flight from Dulles to Dubai, they called the plane back to the gate and removed his luggage.

On international flights, passengers and their luggage must be headed toward the same destination, according to U.S. policy.

Al Murisi also changed his travel plans in Chicago to take a direct flight to Amsterdam, raising suspicion among U.S. officials. Federal Air marshals were on the flight from Chicago to Amsterdam, a law enforcement official said.

The men were arrested Monday morning at Schiphol Airport after getting off a United Airlines flight from Chicago. No charges have been filed against the men in the U.S., a law enforcement official said.

Speaking to reporters on Air Force One as President Obama flew to Fort Bliss in Texas, Deputy White House Press Secretary Bill Burton said Obama has been briefed on the investigation into the suspicious luggage.

Kudwa, the Homeland Security spokeswoman, said once officials found suspicious items in luggage associated with two passengers on Sunday night's flight, they notified the Dutch authorities.

Security at Amsterdam's main airport has been boosted this year, after Umar Farouk Abdulmutallab, a Nigerian student, flew from Schiphol airport to Detroit on Christmas Day with explosives in his underwear. Abdulmutallab allegedly tried to detonate the explosives over the United States before being grabbed by passengers and crew.